They're both the exact same resource mix because it's the exact same grid.
Here's the resource mix https://feature.wecc.org/soti/topic-sections/resources/index...
The difference in cost is entirely contracting and last mile transmission.
https://www.wecc.org/_catalogs/masterpage/wecc/Images/WI-Map...
Also, just because 2 states are interconnected doesn't mean that the interconnects have enough capacity for electricity sources to be fungible.
Edit: After the 2003 blackouts the Eastern interconnect got much more... connected
https://upload.wikimedia.org/wikipedia/commons/d/d4/UnitedSt...
They're both the exact same resource mix because it's the exact same grid.
Here's the resource mix https://feature.wecc.org/soti/topic-sections/resources/index...
The difference in cost is entirely contracting and last mile transmission.